Methods and materials for the catayltic reduction of carbon dioxide

ABSTRACT

The present disclosure is directed to methods of catalytically reducing carbon dioxide, each method comprising: contacting a composition comprising a spinel-type transition iron oxide with an alkali metal carbonate, bicarbonate, or mixture thereof at a first temperature to form CO, and an alkali metal ion-transition metal oxide; hydrolytically extracting at least a portion of alkali metal ions from the alkali metal ion-transition metal oxide by the reaction with CO 2  and liquid H 2 O at a second temperature; and thermochemically reducing the transition metal composition of the second step at a third temperature, with the associated formation of O 2 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/855,180, filed Apr. 2, 2013, which claims priority to U.S. PatentApplication No. 61/621,325, filed Apr. 6, 2012, the contents of whichare incorporated by reference herein in their entirety.

TECHNICAL FIELD

This disclosure relates to the field of energy storage, including thethermochemical production of hydrogen and CO, from water and CO₂,respectively.

BACKGROUND

There is a considerable amount of under-utilized thermal energy fromhigh-temperature heat sources (>700° C.), e.g., nuclear power plants,because the maximum operating temperature for steam turbines istypically below 650° C. (primarily limited by the corrosiveness of highpressure, high temperature steam).

Thermochemical cycles for water splitting or CO₂ reduction are able toconvert thermal energy into chemical energy stored in hydrogen and CO,respectively. Water (CO₂) is split into stoichiometric amounts ofhydrogen (CO) and oxygen in a series of chemical reactions via a closedthermochemical cycle, with heat as the only energy input. No otherproducts are produced in these cycles.

Two types of thermochemical cycles are generally used for this purpose:high-temperature, two-step cycles and low-temperature, multistep cycles.The former usually employs relatively simple reactions and benignchemicals, e.g., transition metals and metal oxides; however, theoperating temperature required to close the cycle is typically higherthan 1500° C. Currently, heat sources with such high temperatures, e.g.,high temperature solar concentrators, are still scarce. In contrast,heat sources at temperature range of 700-1000° C. are much moreabundant, e.g., nuclear power plants and medium-scale solarconcentrators. In addition, high-temperature operating fluids, e.g.,molten salts, have been developed to work in this temperature range.Low-temperature multistep thermochemical cycles are designed to operateat 700-1000° C.; however, the toxic and corrosive chemicals involvedpose significant environmental and engineering challenges. For example,in each reaction of the three-step sulfur-iodine thermochemical cyclefor water splitting, with a highest operating temperature of 850° C., atleast one of the following chemicals are involved: H₂SO₄, HI, SO₂ andI₂.

There is a need for thermochemical water splitting cycles that involvenon-corrosive solids and operate at below 1,000° C. The disclosedinventions address these needs.

SUMMARY

The disclosed inventions take advantage of new cycles that uselow-temperature multistep and high-temperature two-step cycles. Incertain embodiments, a new thermochemical water splitting cycle isprovided that uses non-corrosive solids that can operate with atemperature at or below about 850° C. In other embodiments, theinvention provides a new CO₂ reduction cycle based on analogous methods.

Also provided are methods for thermochemically forming H₂, O₂, or acombination thereof from water, each method comprising: (a) contacting acomposition comprising a spinel-type transition metal oxide of formulaM₃O₄ with an alkali metal carbonate, bicarbonate, or mixture thereof inthe presence of H₂O at a first temperature in a range of from about 450°C. to about 1000° C. to form H₂, CO₂, and an alkali metal ion-transitionmetal oxide, said alkali metal ion-transition metal oxide having anaverage transition metal oxidation state that is higher than the averageoxidation state of the transition metal in the spinel-type transitionmetal oxide; (b) hydrolytically extracting at least a portion of alkalimetal ions from the alkali metal ion-transition metal oxide by thereaction with CO₂ and liquid H₂O at a second temperature in a range offrom 60° C. to 250° C. to form a transition metal composition comprisingan oxidized ion extracted-transition metal oxide in which the averageoxidation state of the transition metal in the oxidized ionextracted-transition metal oxide is the same as the average oxidationstate of the transition metal in the alkali metal ion-transition metaloxide; and (c) thermochemically reducing the transition metalcomposition of step (b) at a third temperature in a range of from 450°C. to 1250° C., with the associated formation of O₂; wherein thetransition metal, M, comprises iron, manganese, or a combinationthereof, and the corresponding spinel-type transition metal oxidecomprises Fe₃O₄, Mn₃O₄, or a solid solution or physical mixture thereof;and wherein the alkali metal ion comprises sodium ion, potassium ion, ora combination thereof.

Also provided are methods for thermochemically forming H₂, O₂, or acombination thereof from water, each method comprising: (a) contacting acomposition comprising a spinel-type Mn₃O₄ with sodium carbonate in thepresence of H₂O at a first temperature in a range of from about 550° C.to about 900° C., preferably about 850° C., to form H₂, CO₂, and asodium birnessite-type A_(x)MnO₂ (0<x<1), preferably derived fromα-NaMnO₂, the sodium birnessite-type manganese dioxide having an averagetransition metal oxidation state that is higher than the averageoxidation state of the transition metal in the spinel-type Mn₃O₄; (b)hydrolytically extracting at least a portion of sodium cations from thesodium birnessite-type manganese dioxide by the reaction with CO₂ andliquid H₂O at a second temperature in a range of (1) from about 70° C.to about 90° C. at ambient pressure or (2) from about 140° C. to about200° C. at a partial pressure of CO₂ in a range of from about 3 bar toabout 20 bar to form a transition metal composition comprising anprotonic birnessite in which the average oxidation state of thetransition metal in the protonic birnessite is the same as the averageoxidation state of the transition metal in the sodium birnessite-typemanganese oxide; and (c) thermochemically reducing the transition metalcomposition of step (b) at a third temperature in a range of from about550° C. to about 900° C., preferably about 850° C., with the associatedformation of O₂.

Still other embodiments include methods comprising: (a) contacting acomposition comprising a spinel-type Fe₃O₄ with sodium or potassiumcarbonate, or a mixture thereof, in the presence of H₂O at a firsttemperature in a range of from about 550° C. to about 900° C.,preferably about 850° C., to form H₂, CO₂, and a sodium- orpotassium-type A_(x)FeO₂ (0<x<1), preferably NaFeO₂ or KFeO₂, thesodium- or potassium-type iron dioxide having an average transitionmetal oxidation state that is higher than the average oxidation state ofthe transition metal in the spinel-type Fe₃O₄; (b) hydrolyticallyextracting at least a portion of sodium cations from the sodium- orpotassium-type iron dioxide by the reaction with CO₂ and liquid H₂O at asecond temperature in a range of (1) from about 70° C. to about 90° C.at ambient pressure or (2) from about 140° C. to about 200° C. at apartial pressure of CO₂ in a range of from about 3 bar to about 20 barto form a transition metal composition comprising Fe₂O₃ or a hydratedform thereof, in which the average oxidation state of the transitionmetal is the same as the average oxidation state of the transition metalin the Fe₂O₃ or a hydrated form thereof; and (c) thermochemicallyreducing the transition metal composition of step (b) at a thirdtemperature in a range of from about 1150° C. to about 1250° C., withthe associated formation of O₂.

Additional methods are provided which comprise: (a) contacting acomposition comprising a spinel-type transition metal oxide of formulaM₃O₄ with an alkali metal carbonate, bicarbonate, or mixture thereof, inthe absence of H₂O at a first temperature in a range of from 450° C. to1000° C. to form CO, and an alkali metal ion-transition metal oxide,said alkali metal ion-transition metal oxide having an averagetransition metal oxidation state that is higher than the averageoxidation state of the transition metal in the spinel-type transitionmetal oxide; (b) hydrolytically extracting at least a portion of alkalimetal ions from the alkali metal ion-transition metal oxide by thereaction with CO₂ and liquid H₂O at a second temperature in a range offrom 60° C. to 250° C. to form a transition metal composition comprisingan oxidized ion extracted-transition metal oxide in which the averageoxidation state of the transition metal in the oxidized ionextracted-transition metal oxide is the same as the average oxidationstate of the transition metal in the alkali metal ion-transition metaloxide; and (c) thermochemically reducing the transition metalcomposition of step (b) at a third temperature in a range of from 450°C. to 1250° C., preferably about 1150° C., with the associated formationof O₂; wherein the transition metal, M, comprises iron, and thecorresponding spinel-type transition metal oxide comprises Fe₃O₄; andwherein the alkali metal ion comprises sodium ion, potassium ion, or acombination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions are further understood when read in conjunctionwith the appended drawings. For the purpose of illustrating the subjectmatter, there are shown in the drawings exemplary embodiments of thesubject matter; however, the presently disclosed subject matter is notlimited to the specific methods, devices, and systems disclosed. Inaddition, the drawings are not necessarily drawn to scale. In thedrawings:

FIG. 1(A) shows the isenthalpic and isentropic lines relating thetemperature of the hydrogen. FIG. 1(B) shows the isenthalpic andisentropic lines relating the temperature of the hydrogen generatingstep and the thermal reduction step with pressure corrections. Partialpressure of water was set at the saturation vapor pressure at 60° C.(ca. 0.2 atm) and the partial pressure of hydrogen was lower than thatof water by a factor of 100. The partial pressure of oxygen in thethermal reduction step was set at 1/100 of the reference pressuregenerating step and the thermal reduction step.

FIG. 2 is a schematic representation of a low-temperature, Mn-basedthermochemical cycle. In the cycle:

Step Reaction Temp (° C.) 1 3Na₂CO₃(s) + 2Mn₃O₄(s) → 4NaMnO₂(s) +2CO₂(g) + 2MnO(s) + Na₂CO₃ 850 2 2MnO(s) + Na₂CO₃(s) + H₂O(g) → H₂(g) +CO₂(g) + 2NaMnO₂(s) 850 3 6NaMnO₂(s) + ayH₂O(l) + (3 + b)CO₂(g) → 803Na₂CO₃(aq) + aH_(x)MnO₂•yH₂O(s) + bMnCO₃(s) + cMn₃O₄(s) 4aH_(x)MnO₂•yH₂O(s) + bMnCO₃ → 850 (2 − c)Mn₃O₄(s) + ayH₂O(g) + bCO₂(g) +0.5O₂(g) Net H₂O(g) → H₂(g) + 0.5O₂(g)In one exemplary embodiment, steps 1, 2, and 4 are conducted at 850° C.and step 3 is conducted at 80° C.

FIG. 3 illustrates the changes accompanying the addition of sodiumcarbonate is essential for low temperature water decomposition on Mn₃O₄.FIG. 3(A) provides thermodynamic estimates showing that Na₂CO₃ enablesoxidation of Mn₃O₄ by H₂O. ΔG for α-NaMnO₂ is adopted from Azad et al.,J. Nucl Mater. 144:94-104 (1987) and the remainder of thermodynamic dataare derived from Lide, D. R., CRC Handbook of Chemistry and Physics,(CRC, New York, 2008), pp 5-42; FIG. 3(B) reflects the production of D₂and CO₂ in the oxidation of Mn₃O₄ to form α-NaMnO₂ by D₂O in thepresence of Na₂CO₃ at 850° C. FIG. 3(C) profiles the water splitting(D₂O) on the mixture of MnO and Na₂CO₃ (2:1) to form D₂ and CO₂ andNaMnO₂. FIG. 3(D) profiles the evolution of D₂ for the 5 cycles tested.Experimental conditions are identical to those in FIG. 3(B).

FIG. 4 provides powder X-ray diffraction patterns used to identifysolids phases in hydrogen evolution steps. (i) Solid collected after thehydrogen evolution step (after step 2); (ii) solid collected afterreacting Mn₃O₄ with Na₂CO₃ at 850° C. (after step 1); (iii) sample from(ii) hydrolyzed in an aqueous suspension in the presence of CO₂ at 80°C. for 3 h; and (iv) sample from (iii) annealed 180° C. in Ar for 1 h.

FIG. 5 provides powder X-ray diffraction patterns used to identifyintermediate phases in hydrolysis of α-NaMnO₂. (i) Hydrolysis ofα-NaMnO₂ in an aqueous suspension at 80° C. for 3 h; (ii) hydrolysis ofα-NaMnO₂ in an aqueous suspension at 80° C. for 3 h with bubbling CO₂and (iii) hydrolysis of α-NaMnO₂ in water vapor and CO₂ underhydrothermal condition at 140° C. for 5 h.

FIG. 6 provides data for the thermal reduction of Na⁺ extractedα-NaMnO₂. FIG. 6(A) shows X-ray diffraction patterns of: (i) Na⁺extracted α-NaMnO₂, and (i) after annealing at (ii) 180° C., (iii) 300°C., (iv) 400° C., (v) 500° C., (vi) 600° C., (vii) 700° C. and (viii)850° C. in Ar for 1 h. FIG. 6(B) provides mass spectral data fromtemperature programmed reaction of: (i) MnO₂, (ii) and (iii) Na⁺extracted α-NaMnO₂ and (iv) MnCO₃.

FIG. 7 illustrates D₂ and O₂ yields from multiple cycles of the Mn-basedthermochemical water splitting system.

FIG. 8 provides X-ray diffraction patterns for Mn₃O₄ recovered after theoxygen evolution step in the recycleability study of Example 7.

FIG. 9 provides an exemplary reactor configuration for examining thehydrolytic extraction steps described in Example 8.

FIG. 10 illustrates the degree of hydrolytic sodium ion extraction fromNaMnO₂, as a function of elevated temperature and pressures, asdescribed in Example 8.

FIG. 11 provide X-ray diffraction patterns for samples from which sodiumions were extracted, followed by thermal reduction at 850° C.

FIG. 12 shows the degree of hydrolytic sodium ion extraction fromNaMnO₂, as a function of time at 200° C. and elevated CO₂ pressure. Theamount of D₂ produced provides another gauge of the effectiveness of theion extraction.

FIG. 13 provides gas evolution data from temperature programmed reactionof Mn₂O₃ with Na₂CO₃ and MnCO₃.

FIG. 14 provides gas evolution data from temperature programmedreduction of the product resulting from the 200° C. extraction of sodiumion from NaMnO₂.

FIG. 15 provides X-ray diffraction patterns for samples, as a functionof time, from which sodium ions were extracted at 200° C., beforethermal reduction.

FIG. 16 provides X-ray diffraction patterns for samples, taken as afunction of time from experiments in which sodium ions were extracted at200° C., followed by thermal reduction at 850° C.

FIG. 17 illustrates (FIG. 17A) the reaction of Mn₃O₄ with Li₂CO₃ (toptrace), Na₂CO₃ (middle trace) and K₂CO₃ (bottom trace) in the absence ofwater; (FIG. 17B) Solids after the thermal treatment at 850° C. in FIG.17A were cooled down to 200° C., before D₂O was introduced. The sampleswere then subjected to a temperature ramp-and-hold treatment to 850° C.in D₂O/Ar (5%/95%). (i, ii) D₂ and CO₂ traces for Mn₃O₄/Li₂CO₃; (iii,iv) D₂ and CO₂ traces for Mn₃O₄/Na₂CO₃; and (v, iv) D₂ and CO₂ tracesfor Mn₃O₄/K₂CO₃.

FIG. 18 shows powder X-ray diffraction (XRD) patterns identifyingintermediate phases in reaction of Mn₃O₄ with alkali carbonates (molarratio 2:3) under various conditions. FIG. 18A shows the results when amixture of Mn₃O₄ and Li₂CO₃ was heated to 850° C. before (bottom trace)and after (middle trace) the introduction of water. The top trace showsthe diffraction pattern of the solid recovered after hydrolyzing LiMnO₂in an aqueous suspension at 80° C. with CO₂ bubbling through for 3 h.FIG. 18B. Physical mixture of Mn₃O₄ and K₂CO₃ at room temperature(bottom trace) and heated to 850° C. prior (middle trace) and after (toptrace) the introduction of water

FIG. 19 shows mass fragmentation analysis showing CO₂ is reduced to COwhen reacting with Mn₃O₄ and Li₂CO₃.

FIG. 20 shows the reaction of Fe₃O₄ with Na₂ ¹³CO₃ confirming thereduction of ¹³CO₂ to ¹³CO prior to the introduction of water.

FIG. 21 shows the percentage of CO of the reduction products (H₂ and CO)and total amount of H₂ and CO formed when the physical mixture ofFe₃O₄/Na₂CO₃ (molar ratio 2:3) reacts i) at 850° C. with 2% CO₂ in thecarrier gas (Ar); ii) at 850° C. in Ar; iii) at 750° C. in Ar; and iv)at 650° C. in Ar. D₂O was introduced after the completion of CO₂evolution from the decomposition of Na₂CO₃ to oxidize the remainingFe(II) species to Fe(III). The dashed line shows the theoretical maximumamount of H₂ and CO can be formed per mole of Mn.

FIG. 22 shows CO₂ reduction with Fe₃O₄/Na₂ ¹³CO₃ in the presence (i),ii) and iii)) and absence (iv)) of ¹²CO₂ in the feed.

FIG. 23 shows the results of water pulse experiments that show that therelative concentration of CO₂ and water controls the contribution ofhydrogen evolution and CO₂ reduction over Fe₃O₄ and Na₂CO₃. The part ofthe data enclosed in the box is used to for the product distributionanalysis.

FIG. 24 shows both CO₂ reduction and hydrogen evolution associated withthe reactions of Fe₃O₄ with Li₂CO₃ and K₂CO₃.

FIG. 25 shows X-ray diffraction patterns of solids collected afterreacting the physical mixture of Fe₃O₄ and alkali carbonates in theabsence and presence of D₂O at 850° C.

FIG. 26 illustrates the thermal reduction of (A) Co₃O₄, (B) Na ionextracted Mn₂O₃ and (C) Na ion extracted Fe₂O₃.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing description taken in connection with the accompanying Figuresand Examples, all of which form a part of this disclosure. It is to beunderstood that this invention is not limited to the specific products,methods, conditions or parameters described and/or shown herein, andthat the terminology used herein is for the purpose of describingparticular embodiments by way of example only and is not intended to belimiting of any claimed invention. Similarly, unless specificallyotherwise stated, any description as to a possible mechanism or mode ofaction or reason for improvement is meant to be illustrative only, andthe invention herein is not to be constrained by the correctness orincorrectness of any such suggested mechanism or mode of action orreason for improvement. Throughout this text, it is recognized that thedescriptions refer both to the methods of preparing the desiredproducts, as well as the use of the products so prepared, and viceversa.

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art, and so forth.

When a value is expressed as an approximation by use of the descriptor“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function. The person skilledin the art will be able to interpret this as a matter of routine. Insome cases, the number of significant figures used for a particularvalue may be one non-limiting method of determining the extent of theword “about.” In other cases, the gradations used in a series of valuesmay be used to determine the intended range available to the term“about” for each value. Where present, all ranges are inclusive andcombinable. That is, references to values stated in ranges include everyvalue within that range.

When a list is presented, unless stated otherwise, it is to beunderstood that each individual element of that list and everycombination of that list is to be interpreted as a separate embodiment.For example, a list of embodiments presented as “A, B, or C” is to beinterpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A orC,” “B or C,” or “A, B, or C.”

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.That is, unless obviously incompatible or specifically excluded, eachindividual embodiment is deemed to be combinable with any otherembodiment(s) and such a combination is considered to be anotherembodiment. Conversely, various features of the invention that are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any sub-combination. It is further noted thatthe claims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for use of suchexclusive terminology as “solely,” “only” and the like in connectionwith the recitation of claim elements, or use of a “negative”limitation. Finally, while an embodiment may be described as part of aseries of steps or part of a more general structure, each said step orpart may also be considered an independent embodiment in itself.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present invention, representativeillustrative methods and materials are described herein.

Various embodiments of the present invention provide methods ofthermochemically forming H₂, O₂, or a combination thereof from water,under catalytic conditions, each method comprising: (a) contacting acomposition comprising a spinel-type transition metal oxide of formulaM₃O₄ with an alkali metal carbonate, bicarbonate, or mixture thereof inthe presence of H₂O at a first temperature in a range of from about 450°C. to about 1000° C. (or to about 950° C., to about 900° C., or to about850° C.) to form H₂, CO₂, and an alkali metal ion-transition metaloxide, said alkali metal ion-transition metal oxide having an averagetransition metal oxidation state that is higher than the averageoxidation state of the transition metal in the spinel-type transitionmetal oxide; (b) hydrolytically extracting at least a portion of alkalimetal ions from the alkali metal ion-transition metal oxide by thereaction with CO₂ and liquid H₂O at a second temperature in a range offrom about 60° C. to about 250° C. to form a transition metalcomposition comprising an oxidized ion extracted-transition metal oxidein which the average oxidation state of the transition metal in theoxidized ion extracted-transition metal oxide is the same as the averageoxidation state of the transition metal in the alkali metalion-transition metal oxide; and (c) thermochemically reducing thetransition metal composition of step (b) at a third temperature in arange of from about 450° C. to about 1250° C., with the associatedformation of O₂; wherein the transition metal, M, comprises iron,manganese, or a combination thereof, and the corresponding spinel-typetransition metal oxide comprises Fe₃O₄, Mn₃O₄, or a solid solution orphysical mixture thereof (preferably comprising manganese and Mn₃O₄);and wherein the alkali metal ion comprises sodium ion, potassium ion, ora combination thereof. In some embodiments, the thermochemical reductionof the oxidized-transition metal oxide results in a regeneration of thespinel-type transition metal oxide of (a).

As described above, each of these steps (a)-(c) is considered anindividual embodiment, as are any combination of those steps (e.g., atleast two of the steps comprising (a) and (b), or (a) and (c), or (b)and (c). Further, any reference to H₂O should be interpreted asincluding isotopes of H₂O, including D₂O.

The net reaction of these embodiments comprises the stoichiometricsplitting of water to hydrogen and oxygen, accompanied by changes in theoverall net oxidation step of the metals. In this context, the term“spinel-type transition metal oxide” is well understood by those skilledin the relevant art as describing a particular crystal latticeconfiguration of metal oxides, having an overall empirical formula M₃O₄.As used herein, particularly with respect to Co, Fe, and Mn, while eachindividual metal center may have a nominal integral value (e.g., +2 and+3), the overall net oxidation state of the average metal center is 2⅔.

The term “alkali metal ion-transition metal oxide” refers to thecompound or composition in which at least a portion of the compound orcomposition of comprises stoichiometric or substoichiometric amount oflattice alkali metal ions. Examples of this in the present contextinclude compositions having a nominal formulae A_(x)MO₂ (0<x<1), where Ais an alkali metal ion and M is the transitional metal ion correspondingto the spinel-type transition metal oxide, and preferably where x ishigher than about 0.5, about 0.6, about 0.7, about 0.8, or about 0.9, oris nominally about 1. NaFeO₂, KFeO₂, and α-NaMnO₂ are examples of suchstructures. Such compositions typically comprise layered structures inwhich the alkali metal ions are positioned between octahedrallycoordinated metal oxide layers. Importantly, the mean oxidation of thetransition metal centers in such compositions are higher than theaverage oxidation state of the corresponding spinel metal oxide; thatis, the action of the alkali metal carbonate or bicarbonate allows foroxidizing the metal while reducing water to form hydrogen. In the caseof NaFeO₂, KFeO₂, and α-NaMnO₂, for example, the nominal oxidation stateof the Fe or Mn is +3. The “portion” of alkali metal ions, in thissituation, is sufficient to stabilize the higher oxidation state. Unlessotherwise specified, “at least a portion” refers to at least 50% of themetal oxide composition or compound contains alkali metal ion as AMO₂,though additional specific individual embodiments include those where atleast about 50%, at least about 60%, at least about 70%, at least about80%, at least about 90%, at least about 95%, or substantially all of themetal oxide composition or compound contains alkali metal ion as AMO₂.

The terms “transition metal composition comprising an oxidized ionextracted-transition metal oxide” or simply “oxidized ionextracted-transition metal oxide” refer to a transition metal oxide thathas a mean metal center oxidation state that is higher than thecorresponding spinel transition metal oxide, preferably and/or typicallythe same or similar to the average oxidation state of that in the alkalimetal ion-transition metal oxide, and which has a level of intercalatedalkali metal ion which, if any, is depleted relative to the amount ofthe alkali metal ion in the corresponding alkali metal ion-transitionmetal oxide. Preferably, the mean oxidation states of the metals of thealkali metal ion-transition metal oxide and the oxidized ionextracted-transition metal oxide are the same and the amount of alkalimetal ions within the oxidized ion extracted-transition metal oxide issubstantially or practically zero. Practical realities of extractionkinetics or structure may compromise this preferred condition.

In certain embodiments of the methods, the step (a) of contacting thecomposition comprising a spinel-type transition metal oxide of formulaM₃O₄ with an alkali metal carbonate, bicarbonate, or mixture thereof isdone stepwise first in the absence and then in the presence of H₂O. See,e.g., the Examples. In other embodiments, the contacting of thespinel-type transition metal oxide of formula M₃O₄ with an alkali metalcarbonate, bicarbonate, or mixture thereof is done in entirely in thepresence of water. In still other embodiments, including those where thespinel transition metal oxide is Fe₃O₄, the step (a) may be doneentirely in the absence of water, in which case, the iron may beoxidized by the carbonate itself (see, e.g., Example 10, Table 1).

Some embodiments provide that the carbonate, bicarbonate, or mixturethereof comprises a carbonate of sodium or potassium. Sodium carbonatemay be preferred in the case of a manganese system, while sodium orpotassium carbonates may be suitable for use with iron systems. In suchcases, the corresponding alkali metal ion-transition metal oxide cancomprise a composition having an empirical formula AMO₂, where Arepresents the alkali metal ion.

The several embodiments of the inventive methods have thus far beendescribed as comprising steps involving at least three separatetemperatures. These temperatures are intended to reflect both statichold temperatures and transitional temperatures. For those embodimentswhere static hold temperatures are employed, such temperatures alsoinclude temperature profiles having ramped heat-up and cool-downconditions. Non-limiting exemplars of such temperature profiles aredescribed in the Examples. In certain embodiments, the first and thirdtemperatures are both higher than the second temperature, and may benominally the same or different. In independent embodiments, the firstand third temperatures are independently in a range bounded at the lowerend by a temperature of about 450° C., about 500° C., about 550° C.,about 600° C., about 650° C., about 700° C., or about 750° C., and atupper end by a temperature of about 1250° C., about 1200° C., about1150° C., about 1100° C., about 1050° C., about 1000° C., about 950° C.,about 900° C., or about 850° C. Specific non-limiting exemplary rangesinclude a range of from about 750° C. to about 850° C. for the firsttemperature and a range of from about 550° C. to about 850° C. (for amanganese system) or about from about 550° C. to about 1120° C. (for aniron system) for the third temperature. In some embodiments with amanganese system, both first and third temperatures are about 850° C. Insome embodiments with an iron system, the first temperature is about850° C. and the third temperature is about 1150° C.

Certain embodiments provide that the second temperature may be belowabout 100° C., about 100° C., or in a range of from about 100° C. toabout 250° C., provided that the conditions allow for the presence ofliquid water and the dissolution of the CO₂ therein. Without intendingto be bound by any particular theory, it appears that the waterintercalate between the metal oxide sheets of the alkali metalion-transition metal oxide, expanding the distance between the adjacentlayers, and improving the mobility of the alkali metal ions. Thepresence of the CO₂ in the water reacts to form carbonic acid whoseprotons replace the alkali metal ions, maintaining the oxidation stateof the ion extracted-transition metal oxide. This may be accomplished atambient atmospheric pressures below about 100° C., for examples in atemperature in a range of from about 60° C. to about 95° C., preferablyabout 80° C., by bubbling CO₂ into the water. It may also beaccomplished at a temperature in a range of from about 100° C. to about250° C., or even higher, provided the temperature is kept below thecritical temperatures of water, and where the CO₂ is present at apartial pressure in a range of from about 1 bar to about 25 bar. Thebalance of temperature and pressure may be adjusted to provide forkinetic advantages. In experiments described in Example 8, it has beenshown that temperatures in a range of about 120° C. to about 160° C.,with suitable CO₂ partial pressures, may represent useful embodiments.

Certain specific embodiments provide for method of splitting water intoH₂ and O₂ based on a spinel-type metal oxide system, including iron andmanganese oxides, preferably based on manganese, using a multistep,low-temperature water splitting cycle that can be operated within atemperature regime about or less than about 850° C. In the case of oneexemplary, non-limiting manganese system, the thermochemical cycle canbe envisioned as consisting of, consisting essentially of, or comprisingfour main steps (FIG. 2):

(i) thermal treatment of a physical mixture of Na₂CO₃ and the spinelMn₃O₄ to produce MnO, CO₂, and the layered compound, α-NaMnO₂ at about850° C.;

(ii) oxidation of MnO in the presence of Na₂CO₃ by water to produce H₂,CO₂, and α-NaMnO₂ at about 850° C.;

(iii) Na⁺ extraction from α-NaMnO₂ by suspension in aqueous solutions inthe presence of bubbling CO₂ at about 80° C.; and

(iv) recovery of Mn₃O₄ by thermally reducing the sodium ion extractedsolid produced in step (iii) at about 850° C. Other variations on thistheme, including combining steps and using different temperatures,extraction methods, and metal oxides are also available, as describedbelow. The net reaction is the stoichiometric splitting of water tohydrogen and oxygen without any by-product. Without being bound by thecorrectness of any particular theory, it appears that the incorporationand extraction of Na⁺ into and out of the manganese oxides are thecritical steps in lowering the temperature required for both thehydrogen evolution and the thermal reduction steps.

The invention, then, teaches methods where the embodiments comprisecompounds based on manganese oxide complexes; for example, wherein thetransition metal comprises manganese; the carbonate, bicarbonate, ormixture thereof comprises a carbonate; and the alkali metal ioncomprises sodium ion. Additional independent embodiments include thosewherein the alkali metal ion-stabilized oxidized-transition metal oxidecomprises a composition having an empirical formula of NaMnO₂, formed byreactions resulting from contacting sodium carbonate and Mn₃O₄. In someof these embodiments, contacting sodium carbonate and Mn₃O₄ reactsaccording to the following stoichiometries:Na₂CO₃+Mn₃O₄→2NaMnO₂+CO₂+MnO; and  (i)2MnO+Na₂CO₃+H₂O→H₂+CO₂+2NaMnO₂.  (ii)As discussed above, the hydrogen generation reactions may be conductedin the presence of water, or in the sequential absence and presence ofwater. For example, the Mn₃O₄ may react with Na₂CO₃ in the absence ofwater, forming NaMnO₂ and MnO; in which the Mn(III) species is extractedfrom Mn₃O₄ through the reaction with Na₂CO₃, leaving the Mn(II) speciesin the form of MnO:Mn₃O₄(s)+Na₂CO₃(s)→2NaMnO₂(s)+MnO(s)+CO₂(g)followed by the oxidation of Mn(II) oxide by water in the presence ofNa₂CO₃, producing hydrogen and NaMnO₂:2MnO(s)+Na₂CO₃(s)+H₂O(g)→H₂(g)+CO₂(g)+2NaMnO₂(s)Generally, under these conditions, the NaMnO₂ is in the form ofα-NaMnO₂.

For these manganese systems, certain embodiments provide that the“cation extraction step” is described in the following terms: theMn(III) species in α-NaMnO₂ cannot be thermally reduced below 1000° C.,whereas the transition from Mn(III) and Mn(IV) oxides to Mn₃O₄ occurredbelow 850° C. Therefore, the sodium cations are suitably removed fromthe manganese oxide in order to close the thermochemical cycle below1000° C. The sodium cations in α-NaMnO₂ may be substituted with protonsby suspending the α-NaMnO₂ in water or impregnated the α-NaMnO₂ withwater, each in the presence of CO₂. As discussed above, this may besuitably done below or above 100° C., provided liquid water is presentand CO₂ is dissolved therein. Without being bound by any particulartheory, it appears that water molecules intercalate into the manganeseoxide layers, increasing the distance between the layers and mobilizingsodium cations. Protons from carbonic acid, formed via the reaction ofCO₂ and water, can exchange with the sodium cations between manganeseoxide layers. When CO₂, and in turn protons, are in excess, almost allsodium cations can be removed from the manganese oxide structure. Adisproportionation reaction may accompany the ion exchange process:2Mn(III)(s)→Mn(IV)(s)+Mn(II)(s)All Mn(IV) and the majority of Mn(III) species may be provided in aproton exchanged birnessite phase. A fraction of the Mn(II) and theremainder of the Mn(III) may be present in an amorphous Mn₃O₄ phase,while the rest of Mn(II) species exists in the form of MnCO₃. Since nonet oxidation or reduction reaction occurs during the ion exchangeprocess, the average oxidation state of the Mn remains +3.

Accordingly, in certain embodiments, the alkali metal ion transitionmetal oxide comprises a composition having a stoichiometry of NaMnO₂ andthe extraction of the Na⁺ from the NaMnO₂ is characterized by astoichiometry:6NaMnO₂ +ayH₂O+(3+b)CO₂→3Na₂CO₃+H_(x)MnO₂ ·yH₂O+bMnCO₃ +cMn₃O₄;  (iii)

wherein a+b+3c=6 and (4−x)·a+2b+8c=18.

In specific embodiments, the transition metal composition of step (b)comprises a protonic birnessite, and the thermochemical reduction ofthis product is done at the third temperature in a range of from about750° C. to about 850° C. In this case, step (c) of the method may becharacterized by a stoichiometry:aH_(x)MnO₂ ·yH₂O+bMnCO₃→(2−c)Mn₃O₄ +ayH₂O+bCO₂+0.5O₂;  (iv)

wherein a+b+3c=6 and (4−x)·a+2b+8c=18.

That is, the solid mixture collected after the sodium cation extractionmay be heated to a temperature in a range of from about 750° C. to about850° C., preferably about 850° C., under inert atmosphere (e.g., Ar orN₂). Note that higher temperatures may also be employed (e.g., upwardsof 1250° C.), but for reasons described above, the lower temperaturesare preferred. Under these conditions, the thermochemical reductionrestores the manganese oxides to the spinel-type Mn₃O₄ phase (with therelease of CO₂ and O₂ in the process), that can be used in the nextcycle.

Other specific embodiments also provide methods of thermochemicallyforming H₂, O₂, or a combination thereof from water, under catalyticconditions, each method comprising: (a) contacting a compositioncomprising a spinel-type Mn₃O₄ with sodium carbonate in the presence ofH₂O at a first temperature in a range of from about 550° C. to about900° C., preferably about 850° C., to form H₂, CO₂, and a sodiumbirnessite-type A_(x)MnO₂ (0<x<1), preferably derived from α-NaMnO₂, thesodium birnessite-type manganese dioxide having an average transitionmetal oxidation state that is higher than the average oxidation state ofthe transition metal in the spinel-type Mn₃O₄; (b) hydrolyticallyextracting at least a portion of sodium cations from the sodiumbirnessite-type manganese dioxide by the reaction with CO₂ and liquidH₂O at a second temperature in a range of (1) from about 70° C. to about90° C. at ambient pressure or (2) from about 140° C. to about 200° C. ata partial pressure of CO₂ in a range of from about 3 bar to about 20 barto form a transition metal composition comprising an protonic birnessitein which the average oxidation state of the transition metal in theprotonic birnessite is the same as the average oxidation state of thetransition metal in the sodium birnessite-type manganese dioxide; and(c) thermochemically reducing the transition metal composition of step(b) at a third temperature in a range of from about 550° C. to about900° C., preferably about 850° C., with the associated formation of O₂.

The methods of splitting water may also use iron oxides, and certainembodiments provide that such methods include the use of spinel typeFe₃O₄; the carbonate, bicarbonate, or mixture thereof comprises acarbonate; and the alkali metal ion comprises sodium ion, potassium ion,or a combination thereof. In some of these embodiments, the alkali metalion-transition metal oxide is NaFeO₂ or KFeO₂, formed by the reactionsbetween Fe₃O₄ and sodium carbonate and between Fe₃O₄ and potassiumcarbonate, respectively. Similarly, still other embodiments includethose where the alkali metal ion is Na⁺ or K⁺ or a combination thereof,and the alkali metal ion-transition metal oxide comprises a compositionhaving a stoichiometry of NaFeO₂ or KFeO₂.

Other specific embodiments also provide methods of thermochemicallyforming H₂, O₂, or a combination thereof from water, under catalyticconditions, each method comprising: (a) contacting a compositioncomprising a spinel-type Fe₃O₄ with sodium or potassium carbonate, or amixture thereof, in the presence of H₂O at a first temperature in arange of from about 550° C. to about 900° C., preferably about 850° C.,to form H₂, CO₂, and a sodium- or potassium-type A_(x)FeO₂ (0<x<1),preferably NaFeO₂ or KFeO₂, the sodium- or potassium-type iron dioxidehaving an average transition metal oxidation state that is higher thanthe average oxidation state of the transition metal in the spinel-typeFe₃O₄; (b) hydrolytically extracting at least a portion of sodiumcations from the sodium- or potassium-type iron dioxide by the reactionwith CO₂ and liquid H₂O at a second temperature in a range of (1) fromabout 70° C. to about 90° C. at ambient pressure or (2) from about 140°C. to about 200° C. at a partial pressure of CO₂ in a range of fromabout 3 bar to about 20 bar to form a transition metal compositioncomprising Fe₂O₃ or a hydrated form thereof, in which the averageoxidation state of the transition metal is the same as the averageoxidation state of the transition metal in the Fe₂O₃ or a hydrated formthereof; and (c) thermochemically reducing the transition metalcomposition of step (b) at a third temperature in a range of from about1150° C. to about 1250° C., with the associated formation of O₂.

It has also been discovered that certain spinel-type transition metaloxides, including Fe₃O₄, when reacted with an alkali metal carbonate,bicarbonate, or mixture thereof in the absence of H₂O at a firsttemperature in a range of from about 450° C. to about 1000° C. (or toabout 950° C., to about 900° C., or to about 850° C.) to form CO, and analkali metal ion-transition metal oxide, said alkali metalion-transition metal oxide having an average transition metal oxidationstate that is higher than the average oxidation state of the transitionmetal in the spinel-type transition metal oxide. That is, instead ofreducing the protons in water to form H₂, the reaction proceeds toreduce CO₂ to CO. See, e.g., Example 10, described herein. Once formed,certain of these reaction products, e.g., NaFeO₂ or KFeO₂, may besubjected to the same hydrolytic extraction and follow-up thermochemicalreduction procedures otherwise described herein, resulting in a methodfor the catalytic reduction of carbon dioxide, according to:CO₂→CO+½O₂with an associated exemplary transition metal cycle comprising:2Fe₃O₄+3Na₂CO₃→6NaFeO₂+CO+2CO₂2NaFeO₂+2H⁺→Fe₂O₃+2Na⁺+H₂O3Fe₂O₃→2Fe₃O₄+½O₂

Exemplary methods for the catalytic reduction of carbon dioxide includethose where method comprises: (a) contacting a composition comprising aspinel-type transition metal oxide of formula M₃O₄ with an alkali metalcarbonate, bicarbonate, or mixture thereof in the absence of H₂O at afirst temperature in a range of from about 450° C. to about 1000° C. (orup to about 950° C., up to about 900° C., or up to about 850° C.) toform CO, and an alkali metal ion-transition metal oxide, said alkalimetal ion-transition metal oxide having an average transition metaloxidation state that is higher than the average oxidation state of thetransition metal in the spinel-type transition metal oxide; (b)hydrolytically extracting at least a portion of alkali metal ions fromthe alkali metal ion-transition metal oxide by the reaction with CO₂ andliquid H₂O at a second temperature in a range of from about 60° C. toabout 250° C. to form a transition metal composition comprising anoxidized ion extracted-transition metal oxide in which the averageoxidation state of the transition metal in the oxidized ionextracted-transition metal oxide is the same as the average oxidationstate of the transition metal in the alkali metal ion-transition metaloxide; and (c) thermochemically reducing the transition metalcomposition of step (b) at a third temperature in a range of from about450° C. to about 1250° C., preferably in a range of about 1150° C. toabout 1200° C., with the associated formation of O₂. In particular ofthese embodiments, the transition metal, M, comprises iron; thecorresponding spinel-type transition metal oxide comprises Fe₃O₄; andthe alkali metal ion comprises sodium ion, potassium ion, or acombination thereof. In other particular embodiments, the carbonate,bicarbonate, or mixture thereof comprises a carbonate and the oxidizedion extracted-transition metal oxide of step (b) comprises Fe₂O₃.

Each of the catalytic systems is robust over repetitive cycles.Accordingly, independent embodiments of the present invention includethose where steps (a) then (b) then (c) of the methods described hereinare performed in order at least 5 times, at least 10 times, at least 50times, or at least 100 times, with less than 10%, less than 5%, orpractically no loss of activity, relative to their initial activities.

The following listing of embodiments in intended to complement, ratherthan displace or supersede, any of the previous descriptions.

Item 1. A method of thermochemically forming H₂, O₂, or a combinationthereof from water, said method comprising: (a) contacting a compositioncomprising a spinel-type transition metal oxide of formula M₃O₄ with analkali metal carbonate, bicarbonate, or mixture thereof in the presenceof H₂O at a first temperature in a range of from about 450° C. to about1000° C. (≦950, 900, 850° C.) to form H₂, CO₂, and an alkali metalion-transition metal oxide, said alkali metal ion-transition metal oxidehaving an average transition metal oxidation state that is higher thanthe average oxidation state of the transition metal in the spinel-typetransition metal oxide; (b) hydrolytically extracting at least a portionof alkali metal ions from the alkali metal ion-transition metal oxide bythe reaction with CO₂ and liquid H₂O at a second temperature in a rangeof from about 60° C. to about 250° C. to form a transition metalcomposition comprising an oxidized ion extracted-transition metal oxidein which the average oxidation state of the transition metal in theoxidized ion extracted-transition metal oxide is the same as the averageoxidation state of the transition metal in the alkali metalion-transition metal oxide; and (c) thermochemically reducing thetransition metal composition of step (b) at a third temperature in arange of from about 450° C. to about 1150° C., with the associatedformation of O₂; wherein the transition metal, M, comprises iron,manganese, or a combination thereof, and the corresponding spinel-typetransition metal oxide comprises Fe₃O₄, Mn₃O₄, or a solid solution orphysical mixture thereof (preferably manganese and Mn₃O₄); and whereinthe alkali metal ion comprises sodium ion, potassium ion, or acombination thereof.

Item 2. The method of item 1, wherein the step (a) of contacting thecomposition comprising a spinel-type transition metal oxide of formulaM₃O₄ with an alkali metal carbonate, bicarbonate, or mixture thereof isdone stepwise first in the absence and then in the presence of H₂O.

Item 3. The method of item 1 or 2, wherein the carbonate, bicarbonate,or mixture thereof comprises a carbonate.

Item 4. The method of any of the preceding items, wherein the alkalimetal ion-transition metal oxide comprises a composition having anempirical formula AMO₂, where A represents the alkali metal ion.

Item 5. The method of any one of the preceding items, wherein at leastone of the first and third temperatures is in a range of from about 750°C. to about 850° C.

Item 6. The method of item 1, wherein the second temperature is (1) in arange of from about 60° C. to about 95° C., preferably about 80° C., atambient atmospheric pressure or (2) in a range of from about 100° C. toabout 250° C., wherein the CO₂ is present at a partial pressure in arange of from about 1 bar to about 25 bar.

Item 7. The method of any one of the preceding items, wherein the thirdtemperature is in a range of from about 550° C. to about 1150° C.

Item 8. The method of any one of the preceding items, wherein thethermochemical reduction of the oxidized-transition metal oxide resultsin a regeneration of the spinel-type transition metal oxide of (a).

Item 9. The method of any of the preceding items, wherein the transitionmetal comprises manganese; the carbonate, bicarbonate, or mixturethereof comprises a carbonate; and the alkali metal ion comprises sodiumion.

Item 10. The method of item 9, wherein the alkali metal ion-stabilizedoxidized-transition metal oxide comprises a composition having anempirical formula of NaMnO₂, formed by at least one reaction resultingfrom contacting sodium carbonate and Mn₃O₄.

Item 11. The method of item 10, wherein the at least one reactionresulting from contacting sodium carbonate and Mn₃O₄ is according to thestoichiometries:Na₂CO₃+Mn₃O₄→2NaMnO₂+CO₂+MnO; and  (i)2MnO+Na₂CO₃+H₂O→H₂+CO₂+2NaMnO₂.  (ii)

Item 12. The method of any of the preceding items, wherein the alkalimetal ion-stabilized oxidized-transition metal oxide comprises α-NaMnO₂.

Item 13. The method of any one of the preceding items, wherein thealkali metal ion is Na⁺ and the alkali metal ion transition metal oxidecomprises a composition having an empirical formula NaMnO₂.

Item 14. The method of item 13, wherein the alkali metal ion transitionmetal oxide comprises a composition having a stoichiometry of NaMnO₂ andthe extraction of the Na⁺ from the NaMnO₂ is characterized by astoichiometry:6NaMnO₂ +ayH₂O+(3+b)CO₂→3Na₂CO₃+H_(x)MnO₂ ·yH₂O+bMnCO₃ +cMn₃O₄;  (iii)

wherein a+b+3c=6 and (4−x)·a+2b+8c=18.

Item 15. The method of any one of the preceding items, wherein thetransition metal composition of step (b) comprises a protonicbirnessite, and the thermochemical reduction of the this product is doneat the third temperature in a range of from about 750° C. to about 850°C.

Item 16. The method of item 15, wherein (c) is characterized by astoichiometry:aH_(x)MnO₂ ·yH₂O+bMnCO₃→(2−c)Mn₃O₄ +ayH₂O+bCO₂+0.5O₂;  (iv)

wherein a+b+3c=6 and (4−x)·a+2b+8c=18.

Item 17. The method of any of the preceding items, said methodcomprising: (a) contacting a composition comprising a spinel-type Mn₃O₄with sodium carbonate in the presence of H₂O at a first temperature in arange of from about 550° C. to about 900° C., preferably about 850° C.,to form H₂, CO₂, and a sodium birnessite-type A_(x)MnO₂ (0<x<1),preferably derived from α-NaMnO₂, the sodium birnessite-type manganesedioxide having an average transition metal oxidation state that ishigher than the average oxidation state of the transition metal in thespinel-type Mn₃O₄; (b) hydrolytically extracting at least a portion ofsodium cations from the sodium birnessite-type manganese dioxide by thereaction with CO₂ and liquid H₂O at a second temperature in a range of(1) from about 70° C. to about 90° C. at ambient pressure or (2) fromabout 140° C. to about 200° C. at a partial pressure of CO₂ in a rangeof from about 3 bar to about 20 bar to form a transition metalcomposition comprising an protonic birnessite in which the averageoxidation state of the transition metal in the protonic birnessite isthe same as the average oxidation state of the transition metal in thesodium birnessite-type manganese dioxide; and (c) thermochemicallyreducing the transition metal composition of step (b) at a thirdtemperature in a range of from about 550° C. to about 900° C.,preferably about 850° C., with the associated formation of O₂.

Item 18. The method of any one of items 1 to 8, wherein the transitionmetal comprises iron; the carbonate, bicarbonate, or mixture thereofcomprises a carbonate; and the alkali metal ion comprises sodium ion,potassium ion, or a combination thereof.

Item 19. The method of any one of items 1 to 8 or 18, wherein the alkalimetal ion-transition metal oxide is NaFeO₂ or KFeO₂, formed by thereactions between Fe₃O₄ and sodium carbonate or between Fe₃O₄ andpotassium carbonate, respectively.

Item 20. The method of any one of items 1 to 8, 18, or 19, wherein thealkali metal ion is Na⁺ or K⁺ or a combination thereof, and the alkalimetal ion-transition metal oxide comprises a composition having astoichiometry of NaFeO₂ or KFeO₂.

Item 21. The method of any one of items 1 to 8 or 18 to 20, said methodcomprising: (a) contacting a composition comprising a spinel-type Fe₃O₄with sodium or potassium carbonate, or a mixture thereof, in thepresence of H₂O at a first temperature in a range of from about 550° C.to about 900° C., preferably about 850° C., to form H₂, CO₂, and asodium-type A_(x)FeO₂ (0<x<1), preferably NaFeO₂ or KFeO₂ the sodium- orpotassium-type iron dioxide having an average transition metal oxidationstate that is higher than the average oxidation state of the transitionmetal in the spinel-type Fe₃O₄; (b) hydrolytically extracting at least aportion of sodium or potassium cations from the sodium- orpotassium-type iron dioxide by the reaction with CO₂ and liquid H₂O at asecond temperature in a range of (1) from about 70° C. to about 90° C.at ambient pressure or (2) from about 140° C. to about 200° C. at apartial pressure of CO₂ in a range of from about 3 bar to about 20 barto form a transition metal composition comprising Fe₂O₃ or a hydratedform thereof, in which the average oxidation state of the transitionmetal is the same as the average oxidation state of the transition metalin the Fe₂O₃ or a hydrated form thereof; and (c) thermochemicallyreducing the transition metal composition of step (b) at a thirdtemperature in a range of from about 1150° C. to about 1250° C., withthe associated formation of O₂.

Item 22. A method of catalytically reducing carbon dioxide, said methodcomprising: (a′) contacting a composition comprising a spinel-typetransition metal oxide of formula M₃O₄ with an alkali metal carbonate,bicarbonate, or mixture thereof in the absence of H₂O at a firsttemperature in a range of from about 450° C. to about 1000° C.(preferably to about 950° C., to about 900° C., or to about 850° C.) toform CO, and an alkali metal ion-transition metal oxide, said alkalimetal ion-transition metal oxide having an average transition metaloxidation state that is higher than the average oxidation state of thetransition metal in the spinel-type transition metal oxide; (b′)hydrolytically extracting at least a portion of alkali metal ions fromthe alkali metal ion-transition metal oxide by the reaction with CO₂ andliquid H₂O at a second temperature in a range of from about 60° C. toabout 250° C. to form a transition metal composition comprising anoxidized ion extracted-transition metal oxide in which the averageoxidation state of the transition metal in the oxidized ionextracted-transition metal oxide is the same as the average oxidationstate of the transition metal in the alkali metal ion-transition metaloxide; and (c′) thermochemically reducing the transition metalcomposition of step (b′) at a third temperature in a range of from about450° C. to about 1250° C., with the associated formation of O₂; whereinthe transition metal, M, comprises iron, and the correspondingspinel-type transition metal oxide comprises Fe₃O₄; and wherein thealkali metal ion comprises sodium ion, potassium ion, or a combinationthereof.

Item 23. The method of item 22, wherein the carbonate, bicarbonate, ormixture thereof comprises a carbonate and the oxidized ionextracted-transition metal oxide of step (b) comprises Fe₂O₃.

Item 24. The method of item 22, wherein the third temperature is about1150° C.

Item 25. A catalytic cycle comprising the method of any of the precedingitems, the steps being performed in order (a/a′) then (b/b′) then (c/c′)at least 5 times, or at least 10 times, at least 50 times, or at least100 times.

EXAMPLES

The following Examples are provided to illustrate some of the conceptsdescribed within this disclosure. While each Example is considered toprovide a specific individual embodiment of composition, or methods ofpreparation and use, none of the Examples should be considered to limitthe more general embodiments described herein.

Example 1 Materials Preparation

Co₃O₄ (99.5%), Fe₃O₄ (95%), Mn₂O₃ (99%), Mn₃O₄ (97%), Li₂CO₃ (99%),Na₂CO₃ (99.5%), and K₂CO₃ (99%) were purchased from Aldrich and usedwithout further treatment. Na₂ ¹³CO₃ (99% ¹³C) was purchased fromCambridge Isotope Laboratories and used without further treatment. Themixture of spinel-type metal oxides (Co₃O₄ Fe₃O₄ or Mn₃O₄) and alkalimetal carbonate (Li₂CO₃, Na₂CO₃, or K₂CO₃) with a molar ratio of 2:3 forthe hydrogen evolution step was prepared by mixing these two powders inan agate mortar under ambient condition. In some experiments, the alkalimetal ions were extracted from the respective alkali metal oxides bybubbling CO₂ (99.997%, 10 cc/min) through an aqueous suspension of thepowder (approximately 5 wt % of solid) for 3 h at 80° C. In otherexperiments, sodium ions were extracted at elevated temperatures andpressures using steel Paar autoclaves, using a reactor configuration asshown in FIG. 9 (see Example 8 below). Hydrothermal treatment ofα-NaMnO₂ was carried out in an autoclave at 140° C. for 5 h with themass ratio of α-NaMnO₂/H₂O/CO₂ (dry ice) being roughly 1/10/10. Thecompound α-NaMnO₂ was placed on a small vessel in the autoclave to avoiddirect contact with liquid water. The powder for the oxygen evolutionstep was obtained by separating the solid by centrifugation and dryingat 100° C. in air.

Example 2 Reaction Tests

All powders (approximately 200 mg) were pelletized (20-35 mesh) beforebeing introduced to a quartz reaction tube with an alumina sheath. Thepellets were supported between two layers of alumina sand (16 mesh) toavoid contact with the quartz tube. Hydrogen and oxygen evolution stepswere tested using an Altamira flow reaction system (AMI-200), and theeffluent gas stream was monitored by an on-line mass spectrometer(Dymaxion 2000). Under typical flow conditions, the flow rate of the gaswas 50 cc/min. Depending on the experiments, the gases used were Ar(99.999%), CO₂/Ar (2%/98%) or D₂O/Ar (5%/95%). In one exemplary hydrogenevolution step, the mixture (Mn₃O₄/Na₂CO₃) was heated from roomtemperature to 850° C. at a ramp rate of 20° C./min under Ar (99.999%,50 cc/min) before water (D₂O) vapor was introduced. Water (D₂O) vaporwas introduced by flowing the carrier gas through a bubbler (50 cc/min)with D₂O at room temperature. D₂O was used instead of H₂O to obtain abetter signal-to-noise ratio of the signal in the water splitting step(m/z=4 for D₂ instead of m/z=2 for H₂). Oxides recovered from the CO₂treatment were heated up at a ramp rate of 20° C./min from roomtemperature to 850° C. in Ar atmosphere.

The conditions for water pulse experiments were similar to the flowreactions described above, apart from pulse introduction of D₂O to thegas stream for 2-15 minutes at the desired sample temperature. Thereduction of the ion-extracted oxides was carried out under a 50 cc/minflow of Ar with a temperature ramp from room temperature to 850-1150° C.at 20° C./min.

Example 3 Characterization

Powder X-ray diffraction (XRD) patterns were collected using a RigakuMiniflex II diffractometer using Cu Kα radiation.

Example 4 Thermodynamic Considerations

The general thermodynamic considerations for a thermochemical cyclecomprising two step are shown in FIGS. 1(A) and (B) for the (i)oxidation of a metal or metal oxide, referred to as Red by water to itsoxidized state, referred to as Ox, with the metal at a higher oxidationstate; and (ii) thermal reduction of the Ox phase back to the Red phase,accompanied by the release of oxygen, according to:T1Red+H₂O→Ox+H₂  [1]T2Ox→Red+0.5O₂  [2]where T1 and T2 are the reaction temperatures where ΔG=0 for Eqs. 1 and2, respectively. From the changes in the Gibbs free energy,ΔG_(r)=ΔH_(r)−TΔS_(r), expressions relating T1 to T2 with either (ΔH⁰_(f,Red)−ΔH⁰ _(f,Ox)) or (ΔS⁰ _(f,Red)−ΔS⁰ _(f,Ox)) held constant(isenthalpic or isentropic, respectively) can be obtained. These FIGS.1(A) and (B) demonstrated that it is unlikely, if not impossible, tosplit water with a two-step cycle where there is complete conversionbetween the oxidized and reduced forms at below 1,000° C. Typicaldifferences in the formation enthalpy (ΔΔH) and entropy (ΔΔS) of the Redand Ox phases for metal/oxide systems were found to be below 400 kJ/moland 50 J/mol/K, respectively (conservative upper bounds). Therefore,only the region above both the lower ΔΔS (entropic) and ΔΔH (enthalpic)lines in FIG. 1(A) (i.e., the upper shaded area labeled“Thermodynamically feasible reason”) was deemed to have practicalrelevance, as it is only in this region that both thermodynamic criteriaare satisfied. The target region for low-temperature thermal watersplitting (lower shaded area, T1, T2<1,000° C.) had no overlap with thepractically accessible region when using a two-step process.Additionally, existing two-step thermochemical water splitting cyclesthat have been reported previously all operate at above 1,000° C. Theconclusion that was derived from these results was that thermochemicalwater splitting cycles accomplished below 1,000° C. would require morethan two steps.

Example 5 Steps 1 and 2: Hydrogen Evolution on Mn₃O₄/Na₂CO₃

The presence of Na⁺ enabled the oxidation of Mn²⁺ in Mn₃O₄ to Mn³⁺ bywater, leading to the formation of α-NaMnO₂, CO₂, and H₂. In the absenceof Na₂CO₃, oxidation of Mn₃O₄ to Mn₂O₃ was always thermodynamicallyunfavorable (ΔG>0) (FIG. 3A, top dotted line). The introduction ofNa₂CO₃ drastically changed the thermodynamics of the oxidation reaction;the ΔG of the reaction decreased with increasing temperature and becamenegative around 250° C. (FIG. 3A, solid line). These thermodynamicestimates were consistent with the experimental observations made in thepresent work in that water did not react with Mn₃O₄ in the absence ofNa₂CO₃ at 850° C., and the amount of D₂ obtained by reacting D₂O (D₂O isused instead of H2O to enhance the signal-to-noise ratio in the productdetection and quantification) with the Mn₃O₄/Na₂CO₃ mixture at 850° C.was equivalent to the amount that would be expected when Mn²⁺ wastotally converted into Mn³⁺ (FIG. 3B). Without being necessarily boundby the correctness of any particular theory, it was hypothesized thatNa₂CO₃ extracted the Mn³⁺ from Mn₃O₄ at 500-850° C. to form α-NaMnO₂,CO₂, and MnO [FIG. 2, step 1; note that CO₂ was observed while heatingthe Mn₃O₄/Na₂CO₃ mixture prior to exposure of water (FIG. 3B)]. Hydrogenwas then formed from the water oxidation of MnO at 850° C. in thepresence of Na₂CO₃ (FIG. 2, step 2). The present work confirmed thatthis step could occur independently at 850° C. (FIG. 3C). The ΔG forboth steps decreases with increasing temperature and becameenergetically favorable above 400° C. (FIG. 3A). Upon the introductionof D₂O, a sharp peak indicating the release of CO₂ was observed. Incontrast, the rate of D₂ evolution increased slowly after the D₂Ointroduction, and reached a plateau after approximately 30 min (FIG. 3Dshows reproducibility). The drastically different kinetics for theevolution of CO₂ and D₂ suggested that step 2 was not an elementarystep. The stoichiometry of the proposed reaction predicted thattwo-thirds of the α-NaMnO₂ and CO₂ should be formed via step 1 and theremaining third via step 2. These amounts were experimentally confirmedby the 2:1 (±15%) ratio of the amount of CO₂ evolved before and afterthe introduction of D₂O (FIG. 3B). Further support for this reactionpathway was provided by the identification of the reaction intermediateMnO by powder X-ray diffraction (XRD) measurements. The XRD pattern ofthe solid obtained after the hydrogen evolution reaction (FIG. 2, steps1 and 2) contained α-NaMnO₂ and a hydrated product α-Na_(0.7)MnO_(2.14)[FIG. 4, i; α-NaMnO₂ can form α-Na_(0.7)MnO_(2.14) when exposed towater]. Importantly, the XRD pattern of the powder collected after step1 clearly showed the presence of MnO in addition to the peaks attributedto α-NaMnO₂ and α-Na_(0.7)MnO_(2.14) (FIG. 4, ii). The diffraction peakscorresponding to MnO persisted after α-NaMnO₂ was fully hydrolyzed inthe presence of CO₂ (FIG. 4, iii); the hydrolysis process of α-NaMnO₂ isdiscussed further below). Furthermore, after annealing the hydrolyzedsample at 180° C. in Ar, the only sharp peaks were those from MnO (FIG.4, iv). The identification of the reaction intermediate MnO stronglysupported the proposed reaction pathway.

Example 6 Step 3: Na⁺ Extraction of α-NaMnO₂

Na⁺ could be efficiently extracted from α-NaMnO₂ via hydrolysis in thepresence of CO₂. Na⁺ extraction was a critical step in closing thelow-temperature thermochemical cycle, since α-NaMnO₂ cannot be thermallyreduced below 1,000° C. The compound α-NaMnO₂ is layered, with Na⁺sandwiched between MnO₆ octahedral sheets. Water can intercalate intothese sheets, expanding the distance between adjacent layers to formsodium birnessite (FIG. 5), as evidenced by the disappearance of thediffraction peak at 16.7° in α-NaMnO₂ and the appearance of the 12.5°peak in birnessite (FIG. 5, i). The mobility of Na⁺ was greatly enhancedin birnessite compared to that in α-NaMnO₂ presumably because the MnO₆sheets are pillared by water, and therefore could easily be exchanged byother cations including protons. Complete Na⁺ extraction from α-NaMnO₂by hydrolysis in acidic conditions to form protonic birnessite (H⁺birnessite) has been reported. See, e.g., Omomo Y., et al., “Preparationof protonic layered manganates and their intercalation behavior,” SolidState Ionics 151:243-250 (2002). Here, this conversion was achieved bybubbling CO₂ through an aqueous suspension of α-NaMnO₂ at 80° C. for 3h. A disproportionation mechanism has been proposed to explain theoxidation state change of Mn in α-NaMn(III)O₂ when it converts into thebirnessite phase with an average oxidation state of 3.5-3.8:2Mn(III)_(solid)→Mn(IV)_(solid)+M(II)_(aqueous)→[CO₂]→Mn(IV)_(solid)+Mn(II)CO_(3 solid):  [3]The Mn(IV) remained in the solid birnessite phase while Mn(II) wasbelieved to dissolve in the aqueous phase. An insoluble Mn(II) salt orother compounds with Mn(II) were expected to form in the CO₂-assistedhydrolysis of α-NaMnO₂. Very weak and broad diffraction peakscorresponding to the MnCO₃ and Mn₃O₄ phases were present in the samplecollected after hydrolysis of α-NaMnO₂ with CO₂ under ambient condition(FIG. 5, ii). However, characteristic diffraction peaks for MnCO₃ andMn₃O₄ phases were observed after hydrolysis of α-NaMnO₂ with CO₂ underhydrothermal conditions (FIG. 5, iii). Accelerated crystal growth underhydrothermal conditions was most likely responsible for MnCO₃ and Mn₃O₄crystals large enough to be detected by the diffraction measurements.The presence of the MnCO₃ and Mn₃O₄ phases provided strong evidence tosupport the disproportionation mechanism (Eq. 3; implies the averageoxidation state of Mn in all Mn-containing solids is still +3).

Example 7 Step 4: Oxygen Evolution by Thermal Reduction of Solids fromNa⁺ Extraction of α-NaMnO₂

Thermal reduction of the mixture formed after sodium extraction ofα-NaMnO₂ (FIG. 6A, i) in Ar at 850° C. allowed for recovery of Mn₃O₄.The layered structure of protonic birnessite collapsed upon heating to180° C. in Ar, presumably yielding amorphous Mn(III,IV)O_(x) (FIG. 6A,ii). The amorphous phase persisted up to 500° C., where broad and weakdiffraction peaks of Mn₃O₄ begin to appear. These results wereconsistent with the temperature-programmed desorption profile of O₂(FIG. 6B, ii), with the onset of the O₂ desorption peak at approximately450° C. The first oxygen desorption peak at approximately 565° C. fromthe mixture was attributed to the thermal reduction of MnO₂ to Mn₂O₃(FIG. 6B, i), indicating the reduction of amorphous MnO₂ to Mn₂O₃. Thediffraction peaks of Mn₃O₄ for the Na⁺ extracted mixture gradually grewmore intense and narrow as temperature increased; however, nodiffraction peaks corresponding to MnO₂ or Mn₂O₃ were observedthroughout the temperature range tested. The oxygen desorption peaks forthe Na⁺ extracted phase above 565° C. did not correspond to thedesorption peaks from the reduction of Mn₂O₃ to Mn₃O₄ at 810° C. (FIG.6B, i); these desorption events were attributed to the solid statereaction between amorphous Mn₂O₃ and MnO present in the mixture. The CO₂desorption peak from the mixture appeared in a similar temperature rangeas the decomposition of MnCO₃ to MnO and CO₂ (FIG. 6B, iii and iv),confirming the presence of MnCO₃. The XRD pattern for the sample afterthermal reduction was almost identical to that of commercial Mn₃O₄,except for a very weak peak at approximately 16° C. corresponding to atrace amount of α-Na_(0.7)MnO_(2.14).

Example 8 Recyclability of the Complete Cycle

The Mn-based thermochemical system described herein exhibited >90% yieldfor both hydrogen and oxygen evolution and showed no sign ofdeactivation during five cycles (FIG. 7). The amount of O₂ released froma thermal reduction of a commercial, crystalline Mn₂O₃ to form Mn₃O₄(FIG. 7, solid black circle) was identical to that released from thethermal reduction presented above, consistent with the recovery of Mn₃O₄in the thermochemical cycle. Furthermore, the XRD pattern of the Mn₃O₄recovered after the oxygen evolution was identical among the five cycles(FIG. 8) and matched the reference diffraction pattern.

A key feature contributing to the recyclability of the Mn-based systemwas the complete shuttling of Na⁺ into and out of the manganese oxides.The Na⁺ incorporation apparently takes advantage of thermodynamicallyfavorable reactions (FIG. 2, steps 1 and 2) to form α-NaMnO₂. The Na⁺extraction step exploited the mobility of Na⁺ in the layered structurewhen intercalated by water, and was further enhanced by the presence ofCO₂, which drove the equilibrium towards the mixture of protonicbirnessite, Mn₃O₄, and MnCO₃. This mixture could be thermally reduced toMn₃O₄ at 850° C., closing the thermochemical cycle. Importantly, thetrace amount of by-product formed by the incomplete Na⁺ extraction wasreintegrated into the α-NaMnO₂ phase in the next cycle, avoiding theaccumulation of a permanent, inactive phase.

Example 8 Step 3: Na⁺ Extraction of α-NaMnO₂ at ElevatedTemperatures/Pressures

Separate experiments were conducted to investigate the ability toextract Na⁺ from α-NaMnO₂, analogous to the experiments described inExample 6. In each case, sufficient water was added to ensure thatliquid water was always present under the reaction conditions. Theseexperiments were conducted at temperatures in a range of 100° C. and200° C. and pressures upwards of 50 bar, under conditions where liquidwater was still present to provide for the hydrolytic extractions. Asshown in FIG. 10, these experiments seemed to show no advantage in usingtemperatures much above about 140° C., the degree of Na⁺ being about thesame at temperatures above this level. Otherwise, the experimentsdemonstrate that the catalytic cycle performs comparably to thatdescribed in Examples 5 to 7, using this method of extraction. See FIGS.11-16. Nearly 90% of the amount of hydrogen formation can be reachedusing the recovered metal oxides (after thermal reduction) via the ionextraction treatment described in this section.

Investigating Various Combinations of Metal Oxides with Spinel Structure(Mn₃O₄, Fe₃O₄ and Co₃O₄) and Alkali Carbonates (Li₂CO₃, Na₂CO₃ andK₂CO₃) in Thermochemical Cycles for Both Water Splitting and CO₂Reduction.

The reactivity patterns of the metal oxide and alkali carbonatecombinations towards water splitting and CO₂ reduction were investigatedand elucidated.

Example 9 Reactions Between Mn₃O₄ and Alkali Carbonates (Li₂CO₃, Na₂CO₃and K₂CO₃)

Mn₃O₄ was reacted with Li₂CO₃ and Na₂CO₃ in Ar at or below 850° C.,releasing CO₂ as the only product in the gas phase; no appreciablereaction occurred between Mn₃O₄ and K₂CO₃ in this temperature range(FIG. 17A, the molar ratio of Mn₃O₄ and alkali carbonates is 2:3). TheCO₂ evolution peaks were at 625 and 850° C. for Mn₃O₄/Li₂CO₃ andMn₃O₄/Na₂CO₃, respectively. In contrast, no detectable amount of CO₂ wasproduced with the Mn₃O₄/K₂CO₃ mixture below or at 850° C. Theseobservations indicated the reactivity of alkali carbonates with Mn₃O₄,gauged by the temperature of CO₂ evolution peak, follows the sequence:Li₂CO₃>Na₂CO₃>K₂CO₃.

Unlike the reaction between Mn₃O₄ and Na₂CO₃ (reaction 1), where allMn(II) species existed in the form of MnO after reaction, Li₂Mn₂O₄ andLi_(0.4)Mn_(0.6)O phases were identified by powder X-ray diffraction(XRD) measurements after the temperature ramp-and-hold to 850° C. forLi₂CO₃/Mn₃O₄ in the absence of water (bottom trace, FIG. 18A). Since theatomic molar ratio of Li to Mn was 1:1 in the starting mixture, a smallfraction of Li-containing phase must not have been detected by XRD. Thisresult could have either be due to the crystal size of the Li-containingphase being below the detection limit of XRD, or due to theLi-containing phase being amorphous. No significant difference in thepowder XRD pattern was observed after heating the K₂CO₃/Mn₃O₄ mixture inAr atmosphere to 850° C. (bottom and middle traces, FIG. 18B),consistent with the lack of CO₂ evolution.

Hydrogen evolution was observed for Mn₃O₄/Li₂CO₃ and Mn₃O₄/Na₂CO₃ at orbelow 850° C., but not for Mn₃O₄/K₂CO₃ (FIG. 17B). The solid after thethermal treatment described in FIG. 17A was cooled down to 200° C., andsubjected to a second temperature ramp-and-hold treatment to 850° C. inD₂O/Ar (5%/95%). D₂ evolution was detected from ˜540° C. and peaks at˜645° C. and for Mn₃O₄/Li₂CO₃ (trace i in FIG. 17B). Little CO₂ wasproduced during reaction (trace ii in FIG. 17B), indicating most of theLi₂CO₃ had reacted with Mn₃O₄ during thermal treatment prior to waterintroduction. Based on the XRD data, only the orthorhombic LiMnO₂ phasewas present after the hydrogen evolution reaction, suggesting all Mn(II)in Mn₃O₄ had been oxidized to Mn(III) (middle trace in FIG. 18A). D₂evolution occurred at 850° C. on Mn₃O₄/Na₂CO₃ (trace iii in FIG. 17B),and the maximum rate of hydrogen evolution at 850° C. was only 1/50 ofthat for Mn₃O₄/Li₂CO₃ (an indication that Li₂CO₃ is more active inpromoting the hydrogen evolution reaction than Na₂CO₃). For bothMn₃O₄/Li₂CO₃ and Mn₃O₄/Na₂CO₃, the total amounts of hydrogen detectedwere close to the theoretical amount expected for the total oxidation ofthe Mn(II) to Mn(III). The concurrent evolution of CO₂ with hydrogensuggested that not all the Na₂CO₃ was consumed in the reaction withMn₃O₄ prior to the water introduction, and this result was consistentwith the Examples above. In addition, the fact that the ratio of theamount of CO₂ produced before and after water introduction was veryclose to 2 indicated that Na₂CO₃ extracts all Mn(III) in Mn₃O₄ but isunable to react with the Mn(II) species in the absence of water.α-NaMnO₂ was formed as the only solid product after the hydrogenevolution reaction. No detectable amount of hydrogen was produced forMn₃O₄/K₂CO₃, suggesting no oxidation of Mn(II) had taken place. CO₂ wasobserved at 850° C. in the presence of water, and its amount was roughlyequal to that expected from the total decomposition of K₂CO₃. UnreactedMn₃O₄ and K-birnessite were identified after reacting with water at 850°C. by XRD. However, not all peaks in the XRD pattern were accounted for,partly due to the hydroscopic nature of the powder, which formed a wetlayer during the time for one powder XRD measurement (ca. 20 min) Thesolid-phase reaction between Mn₃O₄ with alkali carbonate was carried outprior to the introduction of water in order to independently determinethe temperatures at which these reactions take place. In practicalimplementations, these two steps can be combined into one.

Li cation removal from LiMnO₂ could not be achieved under similarconditions to that of sodium cation removal from α-NaMnO₂, i.e. stirringin an aqueous suspension at 80° C. with CO₂ bubbling through for 3 h(henceforth referred to as water/CO₂ treatment). As shown above, the Nacation could be completely extracted from α-NaMnO₂ via the water/CO₂treatment. The XRD patterns of LiMnO₂ before and after the water/CO₂treatment were very similar (top and middle traces, FIG. 18A),suggesting that little Li cation had been removed from LiMnO₂. Moreover,no detectable amount of O₂ was observed when the water/CO₂ treatedLiMnO₂ was subjected to a temperature ramp-and-hold procedure to 850° C.in Ar (confirming the lack of Li cation removal during the water/CO₂treatment).

A fraction of CO₂ formed from the reaction of Mn₃O₄ with Li₂CO₃ wasreduced to CO during the temperature ramp-and-hold to 850° C. in theabsence of water, but not for Na₂CO₃ or K₂CO₃. For both Mn₃O₄/Li₂CO₃ andMn₃O₄/Na₂CO₃, all Mn(II) species in Mn₃O₄ were oxidized to Mn(III) inLiMnO₂ and α-NaMnO₂, respectively, after reacting with water at 850° C.(FIG. 17B). The amount of D₂ produced from Mn₃O₄/Na₂CO₃ was withinexperimental error (±10%) of the theoretical maximum amount; whereasonly ˜70% of the stoichiometric amount of D₂ was detected forMn₃O₄/Li₂CO₃. Quantitative mass spectrometric analysis showed that theremaining ˜30% of Mn(II) was oxidized by the CO₂ released during thereaction between Mn₃O₄ and Li₂CO₃ prior to the introduction of water,producing CO (FIG. 19). In addition to the parent ion of CO₂ (m/z 44),m/z 28 signal was detected as a cracking fragment of CO₂. For pure CO₂,the m/z 28 signal should theoretically trace the parent ion (m/z 44)signal, differing only by a scaling factor. FIG. 19 shows that m/z 28traces exactly m/z 44 by a factor of ˜0.2 at temperatures below 685° C.,beyond which the two traces deviate significantly from each other. Thedifference in the two traces at temperature above 685° C. was attributedto CO produced from the reaction of CO₂ with the Mn(II) species.

Experiment 10 Reactions Between Fe₃O₄ and Alkali Carbonates (Li₂CO₃,Na₂CO₃ and K₂CO₃)

Both hydrogen evolution and CO₂ reduction reactions occurred onFe₃O₄/Na₂CO₃ at or below 850° C. (FIG. 20). Na₂ ¹³CO₃, rather than Na₂¹²CO₃, was used to differentiate the contribution of CO produced fromthe carbonate and the CO₂ in the feed. Three ¹³CO₂ evolution peaks wereobserved during the temperature ramp-and-hold to 850° C. for Fe₃O₄/Na₂¹³CO₃ (molar ratio of 2:3) in Ar. The peak at 165° C. was likely fromthe decomposition of a sodium bicarbonate impurity in the sodiumcarbonate, whereas the two main ¹³CO₂ evolution peaks at 725 and 850° C.were attributed to the reaction of Fe₃O₄ with Na₂ ¹³CO₃. The MS signalfor ¹³CO (m/z 29, corrected for the contribution from the crackingfragment of ¹³CO₂) started to rise at 735° C., and peaked at 850° C.before returning to the baseline. When D₂O was introduced at 850° C., asharp D₂ evolution peak was observed, indicating not all Fe(II) in Fe₃O₄had been oxidized by CO₂. Approximately 85% and 15% of Fe(II) wasoxidized by ¹³CO₂ and D₂O, respectively. Replacing Na₂ ¹³CO₃ with Na₂¹²CO₃ in the initial solid mixture yielded very similar results. Sincethe molar ratio of the starting mixture of Fe₃O₄/Na₂CO₃ was 2:3, thetotal amount of CO₂ released from Na₂CO₃ was 3 times that needed tofully oxidize all the Fe(II) in Fe₃O₄. The fact that ˜85% of Fe(II) wasoxidized indicated that close to 30% CO₂ was consumed in reductionreaction. Approximately 80% of the Fe(II) was oxidized by CO₂ (producedfrom the reaction between Fe₃O₄ and Na₂CO₃) during the temperatureramp-and-hold to 750° C. in Ar. The onset of CO evolution occurred at˜735° C. (FIG. 20), whereas water oxidized Fe(II) to Fe(III) with thestoichiometric production of hydrogen at as low as 560° C. (Table 1).

The remaining Fe(II) after the completion of CO₂ evolution from thedecomposition of carbonates could then be oxidized to Fe(III) by watervia hydrogen evolution reaction. Therefore, the temperature of thermaltreatment of Fe₃O₄/Na₂CO₃ can be used to control the dominant oxidantfor the oxidation of Fe(II) (FIG. 21).

TABLE 1 Onset temperatures for hydrogen evolution and CO₂ reductionreactions Onset temperature (° C.) Sample CO₂ D₂ CO Mn₃O₄/Li₂CO₃ 400 540685 Mn₃O₄/Na₂CO₃ 515 850 — Mn₃O₄/K₂CO₃ — — — Fe₃O₄/Li₂CO₃ 375 510 675Fe₃O₄/Na₂CO₃ 385 500 735 Fe₃O₄/K₂CO₃ 445 500 805

All of the Fe(II) was oxidized to Fe(III) by CO₂ during the temperatureramp-and-hold to 850° C. in CO₂/Ar (2%/98%) (FIG. 21), producing astoichiometric amount of CO with respect to Fe(II). This resultsuggested that the incomplete oxidation of Fe(II) during the temperatureramp-and-hold procedure conducted in Ar was caused by the limited supplyof CO₂. This theory was also consistent with the observation that the MSsignals for ¹³CO and ¹³CO₂ decreased simultaneously (FIG. 20). In orderto deconvolute the contributions of the CO₂ that evolved from thedecomposition of the Na₂CO₃ and the CO₂ in the carrier gas in the CO₂reduction reaction, Fe₃O₄/Na₂ ¹³CO₃ was subjected to a temperatureramp-and-hold to 850° C. in a ¹²CO₂/Ar (2%/98%) atmosphere (FIG. 22).The onset of the ¹³CO and ¹²CO peaks occurred at similar temperature(˜735° C.). The majority of the reduction product initially was ¹³CO,suggesting ¹³CO₂ from Na₂ ¹³CO₃ was preferentially reduced. Theproximity of the newly formed ¹³CO₂ to the Fe(II) species may play a keyrole. ¹²CO became the dominant reduction product after ¹³CO₂ wasexhausted. ¹³CO accounted for approximately 30% of the reduction productand remainder was ¹²CO, with the total amount of CO produced (¹²CO and¹³CO) being close to the amount required to fully oxidize all Fe(II) toFe(III) (within ±10%).

Hydrogen evolution or CO₂ reduction reaction pathways could becontrolled (reversibly) by tuning the relative concentrations of CO₂ andwater. Upon the introduction of the first D₂O pulse to Fe₃O₄/Na₂CO₃ at850° C., the rate of CO production instantaneously decreased by morethan a factor of 10, and D₂ evolution was observed (FIG. 23). Inaddition, the CO₂ concentration also spiked upon D₂O introduction,indicating less CO₂ was consumed in the reduction reaction. The reverseoccurred when the D₂O pulses were stopped, i.e., CO production recoveredinstantaneously and the D₂ signal decreased to baseline. The pathway foroxidizing Fe(II) to Fe(III) could be controlled by the relative amountsof CO₂ and water present until the Fe(II) species was exhausted.

CO₂ reduction and hydrogen evolution reactions also took place onFe₃O₄/Li₂CO₃ and Fe₃O₄/K₂CO₃ (FIG. 24). The CO evolution during thetemperature ramp-and-hold to 850° C. in Ar started at 680 and 815° C.for Fe₃O₄/Li₂CO₃ and Fe₃O₄/K₂CO₃, respectively. Water (D₂O) wasintroduced at 850° C. after the CO signal returns to baseline, and theamounts of D₂ produced account for approximately 20% and 10% of theFe(II) in Fe₃O₄/Li₂CO₃ and Fe₃O₄/K₂CO₃ respectively. The temperaturesfor the onset of the D₂ evolution for Fe₃O₄ and carbonates during thetemperature ramp-and-hold in D₂O/Ar (5%/95%) were all around 500° C.

The powder XRD patterns showed only the AFeO₂ (A=Li, Na and K) phasesafter the temperature ramp-and-hold to 850° C. in Ar (FIG. 25). Theseresults were consistent with the observation that 80% or more of theFe(II) in Fe₃O₄ was oxidized to Fe(III) by the CO₂ from the carbonates.No detectable changes were observed for the solids collected before andafter the introduction of water at 850° C. for all Fe₃O₄ and alkalicarbonate systems.

Sodium and potassium could be removed from the iron oxide structure viathe water/CO₂ treatment, resulting in a hydrated Fe(III) phase, whereasLi cation removal from LiFeO₂ could not be achieved under similarconditions.

Experiment 11 Reactions Between Co₃O₄ and Alkali Carbonates (Li₂CO₃,Na₂CO₃ and K₂CO₃)

No CO₂ reduction was observed during the temperature ramp-and-hold to850° C. for Co₃O₄ with any of the three alkali carbonates (Li₂CO₃,Na₂CO₃ and K₂CO₃) in Ar; nor did hydrogen evolution occur upon theintroduction of D₂O at 850° C. However, Co₃O₄ did react with Li₂CO₃during the temperature ramp to 850° C., as evidenced by the LiCoO₂ andLi_(0.21)CO_(0.79)O phases identified by powder XRD measurement of thesolid collected after the thermal treatment and the sharp CO₂ evolutionpeak at 575° C. A CO₂ evolution peak occurred at ˜850° C. and had a longtail for both Co₃O₄/Na₂CO₃ and Co₃O₄/K₂CO₃ mixtures during thetemperature ramp-and-hold procedure. The total amount of CO₂ evolved wasclose to that expected for total decomposition of Na₂CO₃ and K₂CO₃. Nosodium or potassium-containing phase was identified by XRD after thetemperature and hold procedure for Co₃O₄/Na₂CO₃ and Co₃O₄/K₂CO₃, alsosuggesting the total decomposition of the carbonates. The lack ofcrystalline sodium or potassium containing phases might be attributed tothe hydroscopic nature of sodium and potassium oxides and hydroxides.The X-ray diffraction patterns showed little change after theintroduction of water at 850° C. for all alkali carbonates, consistentwith the lack of hydrogen evolution.

General Discussion of Comparison of Reactivity Among Alkali MetalCarbonates:

The ability of alkali carbonates to promote hydrogen evolution and CO₂reduction with a given metal oxide that has a spinel structure followedthe descending order: Li₂CO₃>Na₂CO₃>K₂CO₃ (e.g., evaluated by the onsettemperatures for hydrogen evolution and CO₂ reduction reactions onvarious combinations of Mn₃O₄ and alkali carbonates (Table 1)). D₂started to evolve on Li₂CO₃/Mn₃O₄ at 540° C., and in contrast nodetectable amount of D₂ was observed on Na₂CO₃/Mn₃O₄ until 850° C. Inaddition, the peak rate of D₂ evolution on Li₂CO₃/Mn₃O₄ is ˜50 timesthat measured from Na₂CO₃/Mn₃O₄. No D₂ evolution was detected forK₂CO₃/Mn₃O₄ at reaction temperature up to 850° C. For the CO₂ reductionreaction, the onset temperatures for CO evolution were 675, 735 and 805°C. for Li₂CO₃/Fe₃O₄, Na₂CO₃/Fe₃O₄ and K₂CO₃/Fe₃O₄, respectively, furtherconfirming the trend in reactivity for the carbonates investigated.Comparison of reactivity among the spinel metal oxides

General Discussion of Comparison of Reactivity Among Spinel MetalOxides:

The reactivity for hydrogen evolution and CO₂ reduction reactions ofspinel metal oxides with a given alkali carbonate followed thedescending order: Fe₃O₄>Mn₃O₄>Co₃O₄. Hydrogen evolution occurred onFe₃O₄ with all three alkali carbonates, whereas Mn₃O₄ had to be combinedwith the more reactive carbonates, i.e., Li₂CO₃ and Na₂CO₃, for theevolution of hydrogen to be observed. For Co₃O₄, hydrogen evolution didnot occur even when it was combined with the most reactive lithiumcarbonate. This reactivity trend was more pronounced for the CO₂reduction reaction, which occurred on all Fe₃O₄ containing sample withalkali carbonates. In contrast, only when Mn₃O₄ combined with lithiumcarbonate, could CO evolution be detected. None of the Co₃O₄ containingsamples was active for CO₂ reduction at or below 850° C.

Experiment 12 Thermal Reduction of the Metal Oxides

After removing alkali cations from the metal oxide structure, the solidswere subject to temperature ramp-and-hold procedures in Ar to thermallyreduce to the spinel oxides (FIG. 26). The alkali cation extractedMn(III) and Fe(III) oxides were fully reduced to Mn₃O₄ and Fe₃O₄ at 850and 1150° C., respectively, similar to the temperatures for thermalreduction of Mn₂O₃ and Fe₂O₃. Co₃O₄ was reduced to CoO during thetemperature ramp of the Co₃O₄/Na₂CO₃ mixture to 850° C., as evidenced bythe powder XRD measurement and the O₂ evolution peak at ˜850° C. forpure Co₃O₄.

The extraction of alkali cations from the alkali metal(III) oxides wasseen as important in being able to close the thermochemical cycle atreasonable temperatures (<1000° C.), since alkali metal(III) oxidescould not easily be thermally reduced (<1500° C.). The degree ofdifficulty for alkali cation extraction paralleled the reactivity of thecorresponding alkali carbonates: Li⁺>Na⁺>K. No appreciable lithiumextraction was observed for LiFeO₂ and LiMnO₂, whereas almost completesodium extraction could be achieved for both NaFeO₂ and NaMnO₂.Furthermore, complete potassium cation removal could also be achieved bythe water/CO₂ treatment from KFeO₂.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description and the examples that follow are intended toillustrate and not limit the scope of the invention. It will beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted without departing from the scope ofthe invention, and further that other aspects, advantages andmodifications will be apparent to those skilled in the art to which theinvention pertains. In addition to the embodiments described herein, thepresent invention contemplates and claims those inventions resultingfrom the combination of features of the invention cited herein and thoseof the cited prior art references which complement the features of thepresent invention. Similarly, it will be appreciated that any describedmaterial, feature, or article may be used in combination with any othermaterial, feature, or article, and such combinations are consideredwithin the scope of this invention.

The disclosures of each patent, patent application, and publicationcited or described in this document are hereby incorporated herein byreference, each in its entirety, for all purposes.

What is claimed:
 1. A method of catalytically reducing carbon dioxide,said method comprising: (a) contacting a composition comprising aspinel-type transition metal oxide of formula M₃O₄ with an alkali metalcarbonate, bicarbonate, or mixture thereof in the absence of H₂O at afirst temperature in a range of from 450° C. to 1000° C. to form CO, andan alkali metal ion-transition metal oxide, said alkali metalion-transition metal oxide having an average transition metal oxidationstate that is higher than the average oxidation state of the transitionmetal in the spinel-type transition metal oxide; (b) hydrolyticallyextracting at least a portion of alkali metal ions from the alkali metalion-transition metal oxide by the reaction with CO₂ and liquid H₂O at asecond temperature in a range of from 60° C. to 250° C. to form atransition metal composition comprising an oxidized ionextracted-transition metal oxide in which the average oxidation state ofthe transition metal in the oxidized ion extracted-transition metaloxide is the same as the average oxidation state of the transition metalin the alkali metal ion-transition metal oxide; and (c) thermochemicallyreducing the transition metal composition of step (b) at a thirdtemperature in a range of from 450° C. to 1250° C., with the associatedformation of O₂; wherein the transition metal, M, comprises iron, andthe corresponding spinel-type transition metal oxide comprises Fe₃O₄;and wherein the alkali metal ion comprises sodium ion, potassium ion, ora combination thereof.
 2. The method of claim 1, wherein the carbonate,bicarbonate, or mixture thereof comprises a carbonate and the oxidizedion extracted-transition metal oxide of step (b) comprises Fe₂O₃.
 3. Themethod of claim 1, wherein the third temperature is about 1150° C.
 4. Acatalytic cycle comprising the method of claim 1, the steps beingperformed in order (a), then (b), then (c) at least 5 times.
 5. Themethod of claim 1, wherein the first temperature is in a range of from450° C. to 950° C.
 6. The method of claim 5, wherein the firsttemperature is in a range of from 450° C. to 850° C.
 7. The method ofclaim 1, wherein at least one of the first and third temperatures is ina range of from 750° C. to 850° C.
 8. The method of claim 1, wherein thecarbonate, bicarbonate, or mixture thereof comprises a carbonate.
 9. Themethod of claim 1, wherein the alkali metal ion-transition metal oxidecomprises a composition having an empirical formula AFeO₂, where Arepresents the alkali metal ion.
 10. The method of claim 1, wherein thethird temperature is in a range of from 1150° C. to 1200° C.
 11. Themethod of claim 1, wherein the thermochemical reduction of theoxidized-transition metal oxide results in a regeneration of thespinel-type transition metal oxide of (a).
 12. The method of claim 1,said method comprising: (a) contacting a composition comprising aspinel-type transition metal oxide of formula M₃O₄ with an alkali metalcarbonate, bicarbonate, or mixture thereof in the absence of H₂O at afirst temperature in a range of from 750° C. to 950° C. to form CO, andan alkali metal ion-transition metal oxide, said alkali metalion-transition metal oxide having an average transition metal oxidationstate that is higher than the average oxidation state of the transitionmetal in the spinel-type transition metal oxide; (b) hydrolyticallyextracting at least a portion of alkali metal ions from the alkali metalion-transition metal oxide by the reaction with CO₂ and liquid H₂O at asecond temperature in a range of from 60° C. to 250° C. to form atransition metal composition comprising an oxidized ionextracted-transition metal oxide in which the average oxidation state ofthe transition metal in the oxidized ion extracted-transition metaloxide is the same as the average oxidation state of the transition metalin the alkali metal ion-transition metal oxide; and (c) thermochemicallyreducing the transition metal composition of step (b) at a thirdtemperature in a range of from 1150° C. to 1200° C., with the associatedformation of O₂; wherein the transition metal, M, comprises iron, andthe corresponding spinel-type transition metal oxide comprises Fe₃O₄;and wherein the alkali metal ion comprises sodium ion, potassium ion, ora combination thereof.